Mr Cameron told the BBC's Andrew Marr Show: "I would say to Boris what I say to everybody else - we will be safer, we will be stronger, we will be better off inside the EU.

"I think the prospect of linking arms with Nigel Farage and George Galloway and taking a leap into the dark is the wrong step for our country.

"And if Boris and if others really care about being able to get things done in our world, then the EU is one of the ways in which we get them done."

Bur Mr Johnson ignored the advice and will now be a leading figurehead for the 'Out' campaign along with six rebel Tory Cabinet Ministers including Work and Pensions Secretary Iain Duncan Smith and Justice Secretary Michael Gove.